Frequency divider



Dec' l5 1942' T. B. GlBBs ET AL FREQUENCY DIVIDER Filed March 12, 1940 80000- 15mn 300M /aaw T m 0 mm m m IN1 1, www@ m o. m ma @f mw m l @wvl W GwG/,M j n MWW6 M M" e @f n A, A, Q w x xl l 6 I l all l n l.. 3 4o o 3 0 5 M". P E A" v. 7/. a 6 l.. /8 L A w 7. l 7 m4 .Ik Ju EA/ 5M 0 F TMMAO a 5/ ZU P/l co ,De- 15, 1942.` T. B. GlBBs ETAL FREQUENCY DIVIDER Filed March 12. 1940 s sheets-sheet 2 TAZ S. 3 w @#2 mlm w i www m Vfur J l. .m f M it, E www JUO@ TMP .E .v

De- 15, 1942. T. B. Glass ETAL FREQUENGY DIvIER Filed March 12, 1940 s sheets-sheet :sv L0 Panzer mcg/mm @www ai" Mmm mme Dec. 1s, 1942 UNITED STATES PATENT/OFFICE i Y* rnsau'illvmna '.lhomas B. Gibbs, Chicago, Morris E. Brown, Oak

Park, and Parker B. Wickham, Chicago, Ill., assignors, by mesne assignments, to George W. lBorg Corporation, Chicago, Ill., a corporation of Delaware pplication March 12, l16.940, Serial N0. 323,578

(Cl. Z50-36) 9 Claims.

The present invention relates in general to frequency dividers, and more in particular to frequency dividers which employ a multivibrator or a plurality of multivibrators in tandem, depending on the ratio between the initial frequency and the nal frequency. The object of the invention is to produce a new and improved device of this character.

A multivibrator is an oscillator comprising two space discharge tubes having their plates crossconnected by condensers to their grids; that is, the plate of each tube is capacity coupled to the grid of the other tube. Such an arrangement will oscillate at a natural frequency which is determined by the value of the coupling condensers and the grid and plate resistors through which the condensers charge and discharge alternately. The tubes pass current alternately and in each tube the change from the conductive condition to cutoff occurs very suddenly. The output of a multivibrator therefore has a distorted wave form and is rich in harmonics.

The utility of a multivibrator asa frequency divider depends on the fact that it may be controlled by impulses or alternating currents of a higher frequency and can be given an operating frequency thereby which is exactly the same as some sub-multiple of the control frequency. The control voltages may be applied over control circuits which connect with the plates or grids of the multivibrator tubes. If the voltages in the control circuits are in phase the multivibrator operates at some even sub-multiple frequency, while if the voltages in the control circuits are displaced T80 degrees in phase from each other the multi vibrator operates at some odd sub-multiple frequency.

One application of the multivibrator as a frequency divider that has been proposed and used to some extent is for the purpose of obtaining a constant low frequency current of 60 cycles or thereabouts which can be used for operating a synchronous motor. Cases are known, as in watch timing apparatus, Where it is necessary to operate a motor at a very constant rate. The commercial power supply available is not sufciently constant for this purpose. On the other hand, the most reliable source of constant frequency, a crystal oscillator, has such a high natural frequency that it is altogether unsuitable for operating a motor. By employing several multivi-brator stages, however, operating at successively lower sub-multiple frequencies, the oscillator frequency can be reduced to a. frequency such as 60 cycles, thus giving an output that can be ampliiled and used for the desired purpose. The complete equipment as described, including the crystal oscillator and chain of multivibrators, constitutes a constant frequency generator having a low frequency output.

Such constant frequency generators of the above type as were manufactured and sold prior to our invention were not satisfactory, as the manufacturing cost thereof was excessive and the maintenance expense was very high. These results were due to a number of reasons which will only be briefly mentioned at this point, but which will be explained fully in the course of the general description of the invention.

One cause for the unsatisfactory results was the lack of knowledge of the principles of multivibrator operation and the consequent failure to so proportion the various parts as to obtain the maximum stability of operation. The principal factors having to do with the operation of a multivibrator are the values oi the plate and grid rel sistors, the value of the plate-to-grid coupling condensers and the voltage of the driving or control currents. For the best results each of these values should have a fairly good range of variation in either direction. For example, the value ci the grid resistors should be so selected that reasonable variations up or down such as are found in commercial resistors will not affect the operation by causing the multivibrator to fall out at the frequency ior which it is designed to operate. Each value, such as value of grid resistors, plate resistors, etc., may be so selected that it has a very wide range of variation by adjusting the other values, but each factor affects all the others, and consequently if the values are so selected that a particular one has the maximum range of variations then the limits on the others are correspondingly reduced. Failure to appreciate this fact and lack of miowledge of the relation between the different factors and of the nature of the effect which a variation in each one has on the others has rendered it impossible heretofore to select the optimum values for all the factors. The

' invention solves this problem and produces a conbility. f

Another reason for the lack oi success hitherto has been the use of condenser drive, or condenser coupling between stages. When a single multivibrator is driven from a source oi' alternating current it is frequently necessary to interpose condensers in the drive circuits, and this has become the standard practice. The same idea has been carried over into the interstage coupling circuits tandem connected multivibrators, where indeedit appeared to have a specialadvantage,

To explain this briey, the coupling between a 360 limiting effect on the feedback of 60 cycle voltag e from the secondgmultivibrator Vto the'flrst In mi"aunei--.-ds, the condenser coupling scheme utilizes the selective action of condensers in transmitting high frequency voltages better than those of low frequency. For these and other reasons condenser drive has been accepted as inherently desirable and essential. We have discovered, however, that while condensers have certain advantages, the use of resistors in the drive circuits.

between tandem connected multivibrators gives far better overall results as regards `stability of operation. 'Ihe advantages of condenser coupling are greatly outweighed by the hitherto unperceived disadvantages which it introduces and which are eliminated by using resistor drive. The reasons for the greatly improved results will be fully explained hereinafter.

It has also been the practice heretofore, so far as known, to tune a controlled multivibrator as closely as possible to the sub-multiple frequency at which it is tc operate, the theory being that the nearer the multivibrator is tuned to its operating frequency the easier it will be to keep it in step with the control voltages. We have discoveredrthat this theory is a mistaken one. In constructing a controlled multivibrator we pay no attention to the natural frequency, but select the parts with a view to maximum limits at the operating frequency. This results in a multivibrator having a natural frequency which isfar lower than its operating frequency. We are able to control such a multivibrator at the desired operating frequency by firing each tube by means of impulses which are amplified in the other tube. This feature is also believed i to be new.

Touching briefly on the practical aspects of the manufacture and operation of standard frequency generators using multivibrators, these generators have been strictly a laboratory product. The various limits have been so close that even the very small variation between different tubes of the same type has caused trouble and has made it necessary to check and readjust the values of condensers and resistors in each multivibrator stage. It hasbeen the exception rather than the rule that a complete generator would divide properly in all stages when first tested, and as to those that did operate there was no assurance that some one or more of the variables was not so close tothe limit that a slight change would render the whole generator inoperative. Under these conditions, each unit had to receive individual attention `in the way of exhaustive testing and adjustment during manufacture thereof, which greatly added to the cost.

From the foregoing it may be surmised that such generators gave trouble in the hands of the customers, which is the fact. The values of condensers and resistors change slightly with the passing of time and the tube characteristics change also. Multivibrators would cease to divide properly after a few months, for unexplained reasons, and the equipment would have to be shipped in to the-factory for test and change in capacity or resistor values at the o'ending stages. In case a tube burned out it happened frequently that the concerned multivibrator would not divide properly after a new tubewas installed, which also--made factory inspection and adjustment necessary. These /diiliculties made the maintenance costs prohibitive.

The present invention/eliminates the difil culties formerlyencountered in the manufacture and-maintenance of constant frequency gener*- ators and other apparatus employing multivibrators, and brings such equipment into the same category with amplifiers, radio receivers, and other standard electronic apparatus that is manufactured on a production basis.

It is an object of the invention therefore to produce a frequency divider using multivibrators that is adapted for quantity production from the usual engineering specifications, and which requires only ordinary testing to detect actual faults in the parts or wiring thereof.

A further object is to produce a frequency divider using multivibrators that can be sold under the usual guarantees as to performance, and with the assurance that no troubles will develop other than those to which electronic equipment in general is subject.

A further object is to produce a controlled multivibrator adapted for use individually or in tandem with other similar multivibrators in which optimum values are assigned to all condensers and resistors and in which each condenser and resistor therefore has such wide limits that ordinary commercial parts can be used without regard to standard variations in values.

The invention and the various features thereof whereby the above noted objects are accomplished, including the features previously mentioned and others, will be described more in detail hereinafter with reference` to the accompanying drawing, in which-,-

Fig. 1 is a circuit drawing of a standard frequency generator constructed in accordance with the invention and employing five multivibrator stages;

Fig. 2 is a circuit drawing of a frequency divider in which two multivibrators are driven in parallel from another multivibrator and in which switching means is employed for causing the multivibrators to operate on either odd or even sub-multiple frequencies;

Fig. 8 is a circuit drawing of apparatus for producing any desired output frequency within a predetermined range, the output frequency being of extreme accuracy;

r Fig. 4 is a graph showing the grid and plate voltage curves in a multivibrator vconstructed in I Fig. 6 is a circuit drawing of a multivibrator and control circuits, and A Figs. 7 to l2, inclusive, are graphs which show 'the various operating limits of a controlled multivibrator constructed in accordance with the invention.

Referring to Fig. l, the-rectangle I0 represents an oscillator of known type comprising a piezoelectric crystal and a space discharge tube. Prefe'ably an amplier tube is associated with the oscillator tube and the complete assembly is commonly referred to as a crystal oscillator. A suitable oscillator Afor the purpose is disclosed in the pending application of Thomas B. Gibbs, Ser. No. 123,994, filed February 4, 1937, Patent No. 2.236,- 532, granted April 1, 1941. I

The first multivibrator MV1 comprises the space discharge device or vacuum tube I3 and circuit connections including resistors and condensers as shown. The tube I3 may be a double triode of the type known to the trade as type 6N7. The two plates are connected to the plus B lead. representing a suitable source of positive direct current potential, through resistors 20 and 2|.

` 'I'he two grids are connected to ground potential through resistors 22 and 23. 'I'he plate of the left hand triode is connected to the grid of the right hand triode through a condenser 25, while the plate of the right hand triode is connected to the grid of the left hand triode through the condenser 24. The cathode is connected to ground. The filament or heater is supplied with current in well known manner, details of which have been omitted in order to simplify the drawings.

The other four muitivibrators MV; MVa, MV4, MVS, comprising tubes l, l5, i6, and il and the associated resistors and condensers, may be identi- `cal with the multivibrator MV1, except as to the value of the plate-to-grid coupling condensers, and hence need not be described in detail. l

Ilhe crystal oscillator i is coupled to the first multivibrator MV1 by means of resistors il and i2, which are connected between the output lead of the oscillator and the two plates of the tube i3. The arrangement provides two drive circuits, in which the driving voltages are in phase, and hence the rst stage multivibrator MV1 is adapted to operate at a frequency which is an even submultiple of the oscillator frequency.

The first multivibrator MV1 is coupled to the second multivibrator MVr by two drive circuits which include the resistors and 2l, respectively. These circuits extend to the two plates at the second stage, and since they connect to the two plates at the first stage, driving voltages will be transmitted from the rst stage to the second stage, which are 180 degrees displaced in phase in the two circuits. Accordingly the second stage multivibrator MVz is adapted to operate at a frequency which is an odd sub-multiple of the frequency at which the first stage multivibrator MV1 operates.

The driving or coupling circuits between the second and third stages and between the third and fourth stages are the same as those between the rst and second stages, and accordingly the third and fourth stage multivibrators MVs and MV4 are adapted to operate at odd sub-multiple frequencies. The two drive circuits lfor the last stage are connected to the same plate at the fourth stage, with the result that the driving voltages in the two circuits are in phase and the last stage multivibrator MVs is adapted to operate at an even sub-multiple frequency.

The output lead 28 is connected to one of the plates at the last stage multivibrator and may include a condenser and resistor of suitable value. The output may be used for any desired purpose. For example, it may bel reduced to a sine wave form and amplified as disclosed in the prior application referred to and used for operating a motor.

The frequencies at which the different stages operate are shown on the drawings, also the division factor at each stage. Thus the oscillator frequency is 90,000 cycles per second, and the first stage multivibrator MV1, dividing by 2, operates at a frequency of 45,000 cycles per second. The second stage multivibrator MV: divides by 45 and has a frequency of 9,000 cycles per second, and so on. The values of the resistors and condensers are given in the following table:

Plate resistors, all stages.. .ohxns 30,000 Grid resistors, all stages dor[5,000 Coupling resistors, all stages do- 250,000 Coupling condensers, first stage mmf Coupling condensers, second stage mmf- 720 Coupling condensers, third stage--- mf- .0038 Coupling condensers, fourth stage -mf .0195 Coupling condensers, fifth stage mf .115

The values given are suitable for use with tubes of the type previously mentioned and with a wide range of plate potential. A suitable potential is 250 volts. It will be seen 'that the plate resistors, grid resistors, and drive or coupling resistors are the same at all stages, which greatly simplifies the manufacturing end. The plate-to-grid coupling condensers are of different values at the different stages. I

The proper value of coupling condensers to use at each stage is determined by reference to the curve shown in Fig. 5, where the condenser' capacity is plotted against frequency on a log`- arithmic graph. Thus to determine the value of coupling condensers to use at the fourth stage,

which is to operate at 360 cycles, we note where v determine the proper condenser value for a frequency of 60 cycles per second, the frequency of 600 is used (600 divided by 10 equals 60) and the corresponding condenser value, .0115 mf., is

multiplied by l0 to give .115 mf., which is the required value.

For frequencies greater than 700 cycles per second a similar procedure may be followed. The third multivibrator MVz, for instance, is required to operate at a frequency of 1800 cycles per second. To find the proper condenser value, we use the frequency value (180 multiplied by l0 equals 1800) and locate the corresponding condenser value, which is .038 mi. This value divided by 10 gives the value of .0038 mf., or the required value.

For frequencies higher than about 2000 cycles per second a correction factor must be introduced, because of the distributed capacity of the circuit. This factor is a constant for any given physical arrangement of apparatus and in the particular multivibrator being described it amounts to about 50 mmf. This is such a very small capacity in comparison with the capacity values shown in Fig. 5 that the curve is a straight line; or rather, the departure from a straight line is so small that it cannot be detected, and the curve is arbitrarily drawn as a straight line. As the frequency is increased, the corresponding condenser values become smaller and the ratio of the constant correction factor to the cendenser value increases. At somewhere between 1000 and 2000 cycles per second, depending on the scale on which the rcurve is drawn, the curve begins to depart from a straight line, but not enough to have much practical signiflcancein view ofthe wide condenser limits which are permissible in a multivibrator constructed in accordance with the in- 5 vention. For instance, the ascertained condenser value for a frequency of 1800 cycles per second is .0038 mf., which is equal to 3800 mmf. Applying the correction factor and reducing this condenser value by 50 mmf., we obtain the value 3750 mmf. 10

The diiferenceis less than the normalpercentageii" 'r ters that-are -fused-in Figrfwhich nwill-enable-w variation in commercial condensers of this size and is well within the permissible limits.l As a matter of fact condensers having a capacity of either .0037 or .0038 can be used, since the actual l5 operating limits are much wider apart.

For the frequencies which are substantially higher than 2000 cycles it is, of course, necessary to make a correction. The required capacity for5 any frequency can be found by first finding the indicated capacity by means of the straight line curve and then subtracting the constant which corresponds to the distributed capacity of the circuit. In practice, however, it is more vconvenient to use a curve which covers the whole g5 frequency range and which has beenl corrected f in the higher ranges of frequency, thus enabling the condenser value for any frequency to be read directly. Such a corrected curve has not been shown for lack of space, but can readily be constructed from the information given in the foregoing.

It will be noticed that in a frequency versus capacity curve such as is shown in Fig. 5 .the product of any frequency and the corresponding capacity is equal to a constant. This holds true, of course, only over the straight line portion of the curve. In the case of the particular curve shown the constant happens to be exactly '7. This constant-can be used in place of the curve, 10 if desired, to calculate the capacity corresponding to any desired frequency. Thus 7 divided by 100 equals .07, which is the capacity value corresponding to a frequency of 100.

It should be clearly understood with respect to 4,',

tained from the curve (or constant) are optimum operating values determined by the invention for controlled multivibrator stages having the other characteristics described, and are not the values which`3correspond to the natural frequencies at the seyeral stages. It has been the practice in the past to tune each multivibrator stage as nearly as possible 'to the frequency at which it is required to operate, the control circuits being relied on to slow down or accelerate each stage to keep it in step with the preceding stage, but we have discovered that greatly improved results are secured if the various stagesV all tend to run too slow and are at all times speeded up or accelerated above their natural frequencies by the control circuits. In designing a controlled multivibrator, that is, one which is to be used as a frequency divider, we therefore entirely disregard the natural frequency at which the multivibrator g5 would operate if uncontrolled, and assign condenser values With a view to optimum operating limits under the controlled condition. The reasons for all this will be more fully explained hereinafter. As a matter of interest, it may be stated that there is a wide discrepancy between the natural frequency of the several multivibrator stages in the described constant frequency generator, Fig. 1, and the frequency at which these stages operate. In the case of the 360 cycle stage,

for example, the natural frequency may be around 200 cycles per second.

Itwill be desirable now to explain the operation oi' one multivibrator stage more in detail in order to bring out certain features of the invention, and in particular to facilitate an understanding of the operation and ladvantages of the resistor drive. For this purpose reference may be had to Figs. 4 and 6. The voltage curves in Fig. 4 are labeled with the same reference letthe discussion to be followed readily. The operation will rst be considered with the understand- \ing that no control voltages are being applied to the control circuits C1 and Cz in F18. 6; that is, the multivibrator is operating at its natural frequency as an uncontrolled oscillator. Although the invention is directed to a controlled multivibrator. it is desirablel to consider the free oscillation state before explaining how the control is effected.v f

In vthe operation of the multivibrator, Fig. 6, -the tubes #l and #2 alternately pass current and become biased far below cutoff, while the condensers E and F alternately charge and discharge. These phenomena, of course, accompany voltage changes on the grids and plates of the tubes, which are depicted in Fig. 4. Curves 35 to 38, inclusive, are the voltage curves for the free oscillating condition. In the following paragraphs the operation `is described briefly, technicalities being avoided as far as possible.

At time T1, the tube #1 is passing current.

` Grid A of this tube has a substantially constant small positive potential. The condenser E is charging over a circuit which includes the grid resistor G (shunted by the cathode grid circuit of the tube), the condenser E, and the plate resistor J of tube #2. Due to the high resistance of J as compared to that of the cathode grid circuit of tube #L the grid A cannot `go very farpositive. See curve 38 at time T1. The plate C of tube #l has a moderate positive potential, which is rising slowly d ue to the discharging of condenser F over a circuit which includes the grid resistor H of tube #2 and the cathode plate circuit of tube #1. The potential on plate C is indicated by the curve 35.

At time T1, the tube #2 is not passing current. Grid B is negative, as shown by curve 3l, but its potential is slowly rising toward zero or ground potential due to the discharging of condenser F. Plate D has a high positive potential, but has not reached the potential of the plus B lead, due to the charging of condenser E through resistor J, and the resulting drop across the resistor.

The describedI conditions obtain until time T2 is reached, when grid- B approaches near enough to groundpotential so that tube #2 can begin to pass current. When tube #2 becomes conductive, the potential on plate D falls, causy ing a fall of potential on -grid A due to the coupling through condenser E. The fall of potential on grid A decreases the current flow through tube #1, which causes the potential on plate C to rise. The rise of potential at .plate C raises the potential on grid B, due to the coupling through condenser F, which increases the current flow through tube #2 and causes a further fall of potential at plate D. A kind of regeneration is thus established, which causes the plates and grids of the tubes to undergo great voltage changes almost instantaneously.

Reviewing these changes briefly, the potential on plate D falls with great rapidity to a value fairly close to ground potential, as indicated by curve 3B at time T1. Condenser E being charged, the grid A is driven to a low negative potential, as indicated by ythe curve 38 at timel Ta. Tube #1 is biased below cutoi and the potential on plate C rises sharply, as indicated by curve 35. 'I'he potential on grid B rises to a low positive value, as shown by curve 31. During the interval from Ta to Ts, condenser E is discharging through grid resistor G and the cathode plate circuit ot tube #2, this discharge circuit accounting for the extreme fall of potential at plate D. As condenser E discharges, the potential on plate D gradually rises toward an intermediate value determined by the drop across resistor J. The voltage ourve'for plate D therefore resembles the characteristic discharge curve of a condenser. During this interval condenser Fis charging over a circuit which includes the grid resistor H (shunted by the cathode grid circuit of tube #2), the condenser F, and the plate resistor I of tube #1. The resulting drop through resistor I limits the potential on plate C, which rises in a manner similar to a characteristic condenser curve as the condenser F charges. AThe discharging of condenser E not only influences the potential on plate D as described, but also the potential on grid A which displays a similar rising characteristic.

At time T5 the potential on grid A approaches' near enough to ground potential so that tube #l can begin to pass current. Another regeneration phenomenon is thus initiated, as a result of which grid A goes positive, and tube #l becomes highly conductive, while grid B goes strongly negative and tube #2 ceases to pass current. The changes which take place are similar to those already described and are shown by the curves 35 to 38, inclusive. The operation continues in this manner, with the two tubes functioning alternately inthe manner described.

The rate at which the regenerative phenomena, sometimes referred to as transients take place is determined by the rate at which the condensers charge and discharge, which in turn is determined by the capacity of the condensers and the value of the grid and plate resistors. Since the condenser circuits also Ainclude tube elements, in one case a cathode grid circuit and in another case a cathode plate circuit, the rate is also influenced somewhat by the 'tube characteristics. This fact helps to explain the diiliculties formerly encountered as the result of changing tubes. It may be noticed also that at times T2 and T5 the grids B and A, respectively, are rising in potential quite slowly, as the grid curves are nearly parallel with the zero axis at these points. This indicates an unstablecondition, in which extremely minute changes can delay or accelerate the firing of the tubes, and explains the somewhat erratic behavior of a multivibrator When oscillating without outside control.

The operation of the multivibrator, Fig. 6, under the action of control voltages applied to the control circuits C1 and C1 will now be explained. It will be obvious that the multivibrator is susceptible to control at the times when the grids are approaching very close to ground potential, that is, at the times T2 and T5, and the general practice heretofore has been to eilect control substantially at these points. For instance, if a positive control voltage is applied to grid B over control circuit C1 at time T1, there will be a tendency for the tube #2 to fire at this point, andsimilarly tube #l can be fired at time Ts by a positive potential on control circuit C2. For various reasons this method of control is unsatisfactory, however. We have discovered that far better results are secured by arranging the circuit constants so that the tubes can amplify at the times when control is to be exercised and by firing each tube by negative impulses on the control circuit associated with the other tube.

The manner in which the foregoing is carried out will be explained with reference to the grid voltage curves 39 and 40 andthe control circuit voltage curves 4l and 42, Fig. 4. The grid `voltage curves 39 and 40 are similar to curves 3l and 38, except that the time interval between successive transients isvgreatly decreased, due to the pronounced dierence between the natural frequency of the multivibrator and its operating frequency. The control circuits C1 and Cn are assumed to be connected to another multivibrator, as shown in the case of the multivibrators at the tlrst two stages in Fig. l, and the voltages impressed on the control circuits are the plate voltages at the preceding stage. The control voltage curves therefore resemble the plate voltage curves such as curve in Fig. 4. Since the two control circuits connect, respectively, with the two plates at the preceding stage, the control circuit voltages are degrees out of phase, and the controlled multivibrator divides by an odd integer. Curves 39--42 show division by 5.

Proceeding with the explanation, at time T1 tube #l is passing current, while tube #2 is nonconductive. The grid Bis at a negative potential, rising toward ground potential, but it has not yet reached sufficiently close to ground potential to enable the tube to beign to pass current. In fact, it has not even approximately reached this confdition, as can be seen from curve 3l. The grid is, however, in a condition where it can be effectively controlled. As can vbe seen from curves 4l and 42, the potential lon control circuit C1 rises at time T1, while the potential on control circuit Cz drops ahnost instantly to a minimum value,

these changes being due to a transient at the preceding stage. It will be convenient to refer to these changes as positive and negative impulses.

The positive impulse on control circuit C1 raises the potentialvon grid B, due to the coupling through condenser F. The negative impulse on control circuit C2 lowers the potential on grid A, due to the coupling through condenser E. Tube #l is passing current at this time and is in amplif fying condition, with the result that an .amplifled rise in potential is produced at plate C, which is transmitted to grid B through condenser F. The twoI impulses therefore produce a cumulative effect on grid B, although the effect of the amplied negative impulse, translated to a positive impulse in tube #1, is much the greater. The potential on grid B is thus raised to ground potential, or sufliciently near thereto so that tube #2 can pass current and a transient takes place.

'I'he foregoing explains rather briefly how grid B is controlled. A more detailed discussion will be given later. Grid A is controlled in a similar way, except that the control is effected by means of a negative impulseon control circuit C1 and a positive impulse on control circuit C1. At a time halfway between times Ta and T3 the voltages in the control circuits are right, but the grid A is still ltoo far negative to enable control to bel effected. At time T3 the impulses received over negative, which prevents the multivibrator from undergoing a transient at this point. Thus the positive impulse on control circuit C1 is amplied in tube #2 (nowpassing current) and is trans- Y mitted to the grid A as a negative impulse. At the same time grid A is receiving a negative impulse over control circuit C2. Tube #l therefore cannot nre time Ts.

When time T4 is reached, the control circuit voltages are again correct for ring tube #1 and grid A has approached near enough to ground potential for the operation to be effected. The negative impulse on control circuit C1 is converted to an ampliiled positive impulse in tube #2, raising the potential on grid A. At the same time grid A receives a positive impulse over control circuit Cz. The rise ingrid potential causes tube #l to pass current and the transient takes place.

'I'he operation continues as described. Atetime Tt the controlling impulses are eiective to lire tube #2 again by raising the potential on grid B. 'Iube #l lires ZVzfcycl'es later, in terms of the control voltages, due to a rise in potential on grid A, and so on.

The manner in which the control voltages function when the controlled multivibrator is operating at an even submultiple may now be briefly explained.. For this purpose the curve C1 should be shifted one-half cycle or 180 degrees to the lett soas to bring the control voltages in the two control circuits in phase.

to time with the positive and negative impulses in the other circuit.

At time Ti, a negative impulse is received on control circuit Cr, which tends to lower the potential on grid B. At the same time, a negative impulse is received overvcontrol circuit C2, which lowers the potential on grid A. The potential on plate C therefore rises, and an amplilied positive impulse is transmittedv to grid B through condenser F. The latter impulse is somewhat reduced by the negative impulse received over control circuit C1, but still has several times the amplitude of the original impulse and is sufficient to cause tube #2 to start.

It will benoticed that,I although the control is exercised in thecase of both odd and even divisions by means of negative impulses which are converted into amplified positive impulses, the

The positive and negais tive impulses in one circuit win then coincide as theoretical effectiveness of the controlldiffers in the vvtwo cases because of the fact that during division by an odd integer the ampliiled impulse is aided by the impulse on the other control conductor, whereas during division by an even integer the amplilled impulse is opposed by the impulse on the other control conductor. This would lead` one to 'suppose that a multivibrator operating Aat an oddfsub-multiple'frequency--isYinher'ntly somewhat more stable than one operating at an even sub-multiple frequency, a supposition which is confirmed by experimental proof. We have found that although the stability limits progressively decrease `with the increasing values of the division integer, a multivibrator dividing by 7 is inherently as stable, if not slightly moreV so, than a multivibrator which is dividing by 6. A multivibrator dividing by 4, however, is fully as stable as one dividing by 5. These facts support the conclusion that the control impulses are amplified to a'considerable extent, at least as much as three or four times. It has been stated previously that the use of re-r sistors in the control circuits in place of ther low capacity condensers'formerly used affords greatly improved operating results. The use of condensers cannot be blamed for all the troubles formerly met with, but we have found, nevertheless, that even if the condenser coupling is given the advantage of all other improvements it is still manifestly inferior to resistor coupling. That is, with all other operating conditions identical, the resistor drive is distinctly better than condenser drive. One reason for this may be pointed out in connection with a somewhat more detailed discussion of the initiation ofl a transient by a negative control impulse.

Reverting to the previous explanation of how a I transient is initiated at tube #2, at time Ti a negwhen the negative potential on grid A is rising,

positive impulses are received over both control circuits. Theimpulse on control circuit C2 tends to raise the potential on grid A. The impulse received over Ci, however, is converted into an ampllled negative impulse by the action of tube #2 and the result is that the potential on grid A is reduced. Tube #l therefore cannot undergo a transient at this time.

At time Ts, negative impulses are again received over both control conductors. The negative impulse on Cz tends to lower the potential on grid A. The negative impulse on C1 is converted into an amplied positive impulse at tube #2, which raises the potential on grid A considerably in spite of the negative impulse received over C2. Thus tube #l undergoes a transient at time Ts. The multivibrator divides by 4 instead of by 5. This is true, of course, only if the grid potential has risen suiciently at time Ts to.. permit the amplied negative impulse on C1 to initiate the transient; and if it has not, then Vthe transientl ative impulse is received over C2, which lowers the potential on grid A and raises the potential on plate C. An amplified positive impulse is thus transmitted to grid B through condenser F, which raises the potential ongrid B. If tube #2 now starts to passa little current, as is assumed to be vthe case, the potential on plate D falls, which further lowers the potential on grid A, which in turn causes a further rise in potential at plate C. Thus another positiveimpulse is ktransmitted to grid B which, due to the time constants of the circuit, will be slightly out of phase with the time of origin of the iirst impulse. The ilrst impulse still persists, however, as can be seen from the shape of the curve 42 at T1, and the second impulse is superimposed on the rst, with the result that the two are added insofar as the eiect on grid B is concerned. Thus the initiation of the transient is insured.

When condenser drive is used the wave shape of the impulses delivered to the controlled multivibrator is altogether different, being substantially as shown in curve 43. Each impulse is exceedingly short and appears on the oscillograph as a line line substantially throughout. When a negative impulse of this character is received at time T1 over control circuit C2, it is converted to an amplified positive impulse in tube #1, and therefore raises the potential at grid B. If tube #2 now passes a small amount o f'current the potential on plate D falls, the potential on grid A falls, and the potential on plate C rises,.with the result that a. second positive impulse is transmitted to grid B as in the previous case. Also as in the previous case, the second impulse is displaced in phase from the first impulse, but whereas in the first case this was immaterial because of the persistence of the first impulse, the phase displacement now causes the second'impulse to occur after the first impulse has disappeared and no cumulative effect is secured. Other impulses y follow, but they are of decreasing amplitude due to losses in the circuit and to the fact that thel7 are not superimposed at any point, and the train of impulses may die out before enough current starts to ow in tube #2 to actually initiate the transient. With resistor coupling, on the other hand, the impulses of the train generated by the starting impulse are cumulative and starting of the tube is insured il it begins to pass any current at all.

It should be understood that in the normal operation of a controlled multivibrator constructed in accordance with the invention the somewhat complicated sequence of operations described in the foregoing does not take place, forth'e amplified positive impulses into which the controlling negative impulses are converted are of entirely adequate amplitude to initiate the transients. It is only when one of the constants such as resistor or capacity value is approaching its limits due to changes in value with time, etc., that the feature becomes of importance. At such times the resistor coupled multivibrator will continue in satisfactory operation while the condenser coupled multivibrator will fail. In other words, the effect of the resistor coupling is to increase the limits and to thereby improve the stability.

The improvement noted is obtained without sacrifice of any other advantages. For example, one advantage that has been claimed for condenser drive is the prevention of phase shifts in the controlled multivibrator, which is accomplished by converting the more or less square topped Waves produced by the controlling multivibrator into very short definitely timed impulses. Such phase shifts might indeed occur if the positive impulses were used, but we employ the negative impulses which are exceedingly sharp and definite in point of time. Phase shift cannot possibly occur therefore. Another claimed aclvantage of condenser drive which was mentioned hereinbefore is the selective action of the condensers in passing the high frequency driving impulses more readily than the low frequency impulses produced at the controlled multivibrator, which reduces the effect of the latter im'- pulses on the preceding stage. To obtain any great advantage in this way rather small condensers mustbe used, which progressivelydecrease in capacity as one proceeds from the lowest frequency stage to the higher frequency stages. At the highest' frequency stage the theoretically correct capacity is so small that it has -a value not far above the distributed capacity in the apparatus, and it is very dicult to secure or maintain a capacity coupling of denite value at this stage. This particular feature of condenser coupling is therefore not of any great importance. We have found that with resistor coupling there is a slightly greater feedback from a controlled stage to the preceding stage than there is when condenser coupling is employed, but the inherent stability of the preceding stage (also resistor drive) is so`much better that the overall stability is greatly improved. In other `7 Words, the stability gain in other directions which results from resistor drive is much moreA than sumcient to compensate for the loss Adue to feedback Before leaving the particular subject of control, the reason why we operate a controlled multivibrator at a frequency much higher than its natural frequency may be further explained. It may be assumed for this purpose that the multivibrator, Fig. 6, is so adjusted or tuned that onehalf period is equal to the time Ti-Tr instead of the time 'F2-T5. 'Ihe multivibrator will then undergo transients automatically at times T1, T4, etc. The natural frequency of the multivibrator cannot be exactly correct, however, and it will tend to oscillate too slow o r too fast. If too slow, it will be speeded up by the control impulses and will be kept in step in the manner already described. Ii it tends to run too fast, it will gain on the control impulses and will iire at times successively farther in advance of times such as time T4 until prevented from gaining any more -by the inhibiting effect of the impulses received at times such as time T3. The multivibrator will then continue to undergo regular transients somewhere between time T3 and time T4 and corresponding times along the curves, or at least one might think it would do so from the considerations pointed out thus far. Indeed with careful -adjustment and under proper conditions a multivibrator can be made to operate in this way; that is, it can be controlled at a lower frequency than its natural frequency.

It will be appreciated, however, that when the multivibrator has such a high natural frequency that it tends to gain on the control impulses, the grids will be in condition for control at times quite far in advance of the times at which the transients take place, which means that there is danger of skipping a whole cycle of control voltage. In other words, the multivibrator is on the border line of the next lower division and if dividing by 5 will be in danger of changing over to a division lby 3. This is a condition that is brought about by the fact that the set of inhibiting impulses which are relied on to slow down the controlled multivibrator` are always preceded bya set of impulses which will cause it to ilre if conditions are right. The margin is rather close in any event and with strong drive impulses such as we prefer to use for other reasons is practically non-existent. We therefore operate controlled multivibrators at a considerably higher frequency than their natural frequency so that the control impulses are always employed for positively initiating the transients.

Attention may now be directed more specifically to the limits feature of the invention, which is of primary importance in constructing a multivibrator characterized by stability and freedom from trouble under changing conditions. As previously indicated, in a multivibrator embodying the invention the different parts such as resistors and condensers are correlated with-the object of securing the optimum limits for each. In other words, the values of the dierent parts are selected with a view to the vbest overall operating stability and no part is unduly favored as to its limits to the detriment of any of the other parts.

To enable this feature to be better understood reference may be made to Figs. 7 to 12, inclusive, which grows graphs such as may be used to investigate the operating limits of a multivibrator.

1 given in connection with Fig. 1.

plotted against different plate-to-grid coupling v condenser values. To determine the points on the curves, a standard multivibrator constructed according to the invention and adapted to operate at a frequency of 200 cycles per second is driven from a similar multivibrator operating at 1000 cycles per second and driven from any suitable constant frequency source. The 200 cycle multivibrator divides by 5. 'I'he plate resistors and interstage coupling resistors of the 200 cycle multivibrator have the values which have been Two variable capacities are used as the plate-to-grid coupling condensers and two variable resistances function as grid resistors. An oscillograph is used to\"de termine the frequency at which the controlled multivibrator operates,V the vertical deectingf then ascertaining the range of grid resistor values.

within which the multivibrator will divided by 5. For example, if the condenser value is .035 mf., the value of the grid resistors has to belincreased to 30 thousand ohms before the multivibrator will begin to divide by 5. In a. range oi' grid resistor values below 30 thousand ohms it divides by 3. Asthe value of the grid resistors is increased beyond 30 thousand ohms-the multivibrator continues to divide by until the resistor values exceed 160 thousand ohms, when the multivibrator again begins to divide by 3. Other points on the curves are found in the same way, and after a sufficient number of points have been found the curves can be drawn in. From the manner lin whichl the points are determined, it will be understood that the two curves which `form closed loops within the figure define an area. Within which the multivibrator will divide by 5. With condenser and resistor values which sister values are plotted against condenser values. As in the previous twocases, thearea defined by the two curves is the division by 5 area.

In Figs. 9, 11 and 12 the grid resistor values, l

delivered at the plates or grids of the controlled e multivibrator, but it is inconvenient to measure these voltages. The value of the interstage coupling resistors is a function of the delivered drivek voltages, but coupling resistors of low value can-A not be used 'because of the increased feedback into the preceding stage. In this dilemma resort is had to an expedient which is effective for the purpose. It is known that ample drive is desirable from the standpoint of the driven multivibrator. The drive is increased by decreasing the size of the interstage coupling resistors,'but it has been determined experimentally that the value of these resistors cannot be reduced very 'condenser value of .05 mf. and a resistor value of 100 thousand ohms lies in the division by 7 area. The signiilcanceof the shape and size of the ,division by 5 area will be commented on later.

'I'he curves in Fig. 8 are made in the same way as just described, except that in the case of Fig. 8 the grid resistor values are plotted against plate resistor values and the points on the curves are determined by ascertaining the grid resistor limits within which the multivibrator will divide by 5 for different plate resistor values. The coupling condensers, of course, have a value of .035 mf. With values of grid and plate resistors which determine any point within the area dened by the two curves the multivibrator will divide by 5.

The curves in Fig. l0 are also made in the same way as those in Fig. 7, except that plate remuch below 250 thousand ohms without deleterious effect on the preceding multivibrator. A value of coupling resistor such as 250 thousand ohms is therefore selected arbitrarily and the grid resistor limits (or other limits) are determined by the method illustrated in Fig. 7. The controlled multivibrator is then run from a variable voltage generator and the applied voltage is adjusted until the grid resistor limits are the same as before, at which time it may be assumed that the voltage of the generator is the same as the effective voltage of the multivibrator previously used. In this way it may be determined that the effective voltage applied to the control circuits by a multivibrator operating with a plate voltage of 250 volts is about volts. V'Ifhe voltage of the variable voltage generator may then be adjusted to diiferent values above and below 100 volts and for each value the grid resistor limits maybe determined', thussiixing the l location of points from which the heavy line curves in Fig. '9 are constructed. Needless to say, the generator must be of such a character that it is not affected by the feedback from the multivibrator.

The curves shown in Figs. ll and 12 are constructed in the same way, except that the limits of the plate resistor values and the condenser values, respectively, are determined.

The curves shown in Figs. 9, 1l and 12 could be constructed lin other ways. For instance, means could be arranged for varying the plate voltage at the driving multi-vibrator, which would vary the drivevoltages applied to the control circuits. This method will give equivalent results over a considerable range. The method first described, however, will be found satisfactory.

The curves shown in the several Figures 7 to 12 are typical limit curves for a multivibrator such asdescribed herein and illustrated'in Fig. 1, and show that al1 the limits are ample, perhaps not absolutely the best that could be obtained, but -entirely satisfactory. In the case of each figure, the point determined by the recommended values of the variables is indicated vby a small circle. A. consideration of the location of these circles will enable one to appreciate the meaning and utility of the curves.

To illustrate, in Fig. 7 the location of the circle is determined by the value of grid resistor used in practice, which is 75 thousandohms, and by the capacity value which corresponds to a frequency of 200 cycles per second, which the curve in Fig. shows to be .035 mf. The point thus located is well within ,the division by 5 area in all directions, thus showing excellent limits for both condensers and grid resistors. The capacity can in fact be varied from about .027 mf. to above .042 mf. without changing the division factor, while the value of grid resistors can be varied between 30 thousand and 160 thousand ohms. These are much larger variations than are found in commercial condensers and resistors. It will be noted that the coordinates of the circle are such that it is located in that part of the division by 5 area which lies below the tip of the division by '7 area rather lthan in one of the side branches. which we. have found to be an important consideration.

In Fig. 8 the circle representing the correct operating values is located by the coordinates 30 thousand and 75 thousand, which correspond to the recommended values for the plate and grid resistors, respectively. lAs in Fig. 7, this location is well within the division by 5 area and is approximately the best location as regards the limits of the variables concerned. The plate resistor range is from 16 thousand ohms to 44 thousand ohms, while the grid resistor range is from about 35 thousand ohms to over 150 thousandohms.

In Fig. 10 the circle is again located by the coordinates which correspond to the recommended values, 30 thousandohms for the plate resistorsand .035 mf. for the coupling condensers. These coordinates locate the circle advantageously within the division by 5 area. The limits are larger than would appear at rst glance, since a different scale is used, and are approximately the same as the corresponding limits shown in Figs. 'l and 8. That is, the coupling condenser limits are approximately the same, whether plotted against the grid resistors or the plate resistors, while the limits for each resistor are about the same, whether plotted against the other resistor or the condensers.

In Figs. 9, l1 and 12 another factor, the drive voltage, is introduced. and the curves dene areas which indicate the limits of resistor and condenser values for different drive voltages. As

indicate the recommended values for the two factors concerned in the case of each figure and represent approximate optimum values,

Discussing Fig, 9 briefly, it has been explained previously that it is not desirable to` use resistors in the control circuits which havefa value of much less than -250 thousand ohms, in view of the diflculties which would be encountered due to feedback between stages. This is a somewhat arbitrary limit that is placed on the resistors. They could be increased in value, if there would be any advantage in doing so, but they cannot be decreased in value appreciably. As also previously explained, the applied drive voltages actually obtained at thel driving multivibrator is about 100 volts. This value is therefore taken as one of the coordinates, the other being determined by the grid resistor value, which is 75 thousand ohms. excellent position within the division by 5 area, which as previously mentioned is defined by the heavy line curves.

'Ihe circles are located in Figs. 11 and 12 in the same way, taking 100 volts as the drive volt- The circle thus located has an r lill the

age in each case, the val 'e of 3o thousand ohms for the plate resistors inI Fig. 11,4 and the value of .035 mf. for the condensers in Fig. 12. Again sitins of the circles within the division by 5 areas indicate highly vsatisfactory limits. It is possible that the value ofl 100 volts for the drive voltage represents a slightv compromise, as will be noticed by comparison of Figs. 9 and 12. The first of these gures seems to indicate that the limits as regards the grid resistors could be improved by reducing the drive slightly, which could be done by using somewhat larger coupling resistors. Fig. 12, however, shows that any reduction in .drive would reduce the limitson the 4coupling condensers. The limits shown are quite satisfactory as a matter of fact, and there is little to be gained byany change.

coordinated in the design reliable results cannot be attained.

Fig. 9 also shows a set of curves in dotted lines which were made under the same conditions as the curves in Fig. 7, except that condenser tdrive was substituted for resistor drive. That is, the

resistors in the interstage control circuits were removed and replaced by condensers. Grid resistor values are plotted against coupling condenser values, as in Fig. 7, but the resulting curves are shown in Fig. 9 rather than in Fig. 7 to avoid possible confusion.

It will be noted from the location of the circle (coordinates r.035 mf. and 75 thousand ohms) that fairly good limits are shown, but it will be seen also that the division by 5 area is dened on each side by two lines instead of one. Each pair of lines defines an area in which the multivibrator may or may not divide by 5. There is no clean-cut line of .division between the dinerent adjacent division areas, and results in general are not reproduceable. This is a characteristic of curves made when using condenser drive and helps to explain its unsatisfactory operation in practice.

Reference may now be made to Fig. 2, which shows two multivibrators MV1 and MVs driven in parallel from a precedingv multivibrator MVa, each multivibrator being provided with means for switching the control circuits to enable reither an ,odd or even division to be made. The object of the arrangement is to secure outputs of different frequencies at will.

The reference characters and 5I indicate two control conductors carrying control voltages which are 180 degrees out of phase and having a frequency of '720 cycles per second, These control voltages may be derived from a preceding multivibrator operating at a frequency of 720 cycles per second. The arrangement may include a crystal oscillator operating at a frequency of 90,000 cycles per second and three multivibrator stages each dividing by 5.

The multivibrator MVs comprises a double triode 52 and associated resistors and condensers,

as shown. 'Ihe two control circuits include the resistors 55 and 56. One control circuit is permanently connected to incoming control conductor 50, while the other control circuit is provided with a switch Si by means ofwhich it may be connected to either conductor l0 or ccn-` ductor 5l.

' and on the position of switch Sr.

.rire muitivibreier im comprises message.

associated resistors and condensers. The control circuit including-resistor Il is connected to conductor. I1' outgoing from MV.. The control circuit which includes r` ll is arranged by means ofa switch Si so that it ecn be connected either to conductor I1 or to conductor Il.

erating at their optimum frequencies. The' multfv'ibrator MV1, for instance, when operating at frequencies of V60 'or 90 cycles vper second, will obviously have poorer limits'than when operating atan intermediate frequency of 72 or 80 cycles per second, but the fact that it will operate satis- 'rire multivibrator Mv., including the tubev u, 1 Y

is similar tothe multivibrator MV1 and has a similar arrangement of control circuits includingthe switch Sa.

As previously mentioned, the switches-which l. vare associated with the control circuits of the multivibrators permit the same to be controlled so as te divide edd er even es desired. ffn the Y case of switch Si, for instance, when the switch is in the position shown, the voltages in the control circuits of MVa will be in phase, since both factorily overrthis wide rangeindicates the wide limits which it has at its bestnormal operating frequency. 4We believe that the wide range covered can bev attributed in `a large measure to resistor 'coupling between stages, as we have not been able to obtain at all comparable results with condenser coupling.

I'he 'arrangement oi' Fig. 2 in which two multi- .vibrators'such as MV1 and MV. are driven in parallel from a preceding stage is also. useful without the switching arrangements for changing the division fromodd to even, or vice versa.

control circuits are connected to conductor Il,`

and multivibrator MVe will divide even. Switch Se, however, isshownin the proper position to cause an odd divisionv at MV1, and the same is true as regards switch Ss' and MVr.

'riie multivibrator Mve divides either by 2 or by 3 andto this end has a, normal operating frequency asdetermined by the size of the coupling condensers of approximately 330 cveles per l second. 'I'he natural frequency is below 240 cycles per second. the frequency at which it operates when dividing by 3, so that the control impulses exercise control by speeding up the multivibrator, regardless of the rate of division. The l multivibrator MV1 divides by 3, 4, or'5, dependi lpositions of the switches Si and Si. The numeral which is found in parenthesis before each frequency value indicates the rate of division.

I MV. I MV1 input 2 4 9o 7m 360 g5; 72 (a) 24o (4) so i (a) so For instance, MV1may be arranged permanently for even division by 6, while MV: is arranged for odd division` by 5, thus giving highly useful frequencies of 60 and '12 cycles per second. The same result could be secured by employing only one -multivibrator and a switch for causing it to divide by either 5 or 6, but the former arrangenient has wider limits for each multivibrator and is more reliable..

It mightv be' expected that -where two multivibrators are driven in parallel from a preceding stage, as in Fig. 2, there would be an interaction or coupling' between the two driven stages, which would prevent them lfrom dividlngvat different rates,l as by 5 and 6. This indeed is the case when condenser coupling is employed between the driving and-driven stages, and buffer tubes have to be inserted. Withreslstor coupling, however, we have found no appreciable difference in f, the operation at the driven stages, aseach stage position shown, MVs divides by 2 and MV1 divides by 5, giving an output of 72 cycles per second. If switch Si is changed to its alternateposition, MV1 divides by 4, giving a 90 cycle output. If switch Si is changed to its alternate position, MVs divdesby 3 and operates at 240 cycles per second instead of 360 cycles per second. MV1 then divides either by 4 or 3, depending on theposition of switch Si, and has an output of either or 80 cycles per second.

The multivibrator MV may be the same'as MV1, with the result that two desired frequencies such as 90 and 72 cycles per second, or 60 and y80 cycles per second, can be obtained simultaneously.

The arrangement of Fig. 2 illustrates the highi limits which the multivibrators have when opoperates normally the same as it does when driven separately'. 'At the driving stagethere is a slight reduction in the limits, probably due to increased feedback when two multivibrators are being driven instead of one.

Referring now to Fig. 3, the equipment there shown may be considered as embodying a further development of the basicprinciples disclosed in Fig. 2. Switching apparatus is used for simultaneously shifting the control circuits for odd or even division and for varying the coupling.

capacitybso as to cause a 4multivibrator to divide overl a range extending from 2 to 9. The 'specific object of the arrangement shown is to secure any desired output frequency within a range of 40 to 250 cycles per second'and with an extremely high degree of accuracy.

The reference character 'l5 represents a beat frequency oscillator, the output frequency of which is adjustable over a range of 250-to 500 cycles per second. Within these limits the accuracy of the output is satisfactory percentage Wise, but at a lower frequency the error which may occur in either of the two oscillators used to produce the beat frequency bears too high a. ratio. to the output frequency, and the ratio becomes progressively worse as the beat frequency decreases. For example, an error of one part in 100,000 produces an errorrof one part in 500 for a beat frequency output of 500 cycles, which ls reduced to one part in when the beat frequency is reducedto 100 cycles. In this situaftion the range `of the beat frequency may be limited -to 250500 cycles, within which it has the necessary accuracy, and a multivibrator is em-v 40-250 cycles, thus maintaining the accuracy?` which control voltages maybe impressed on the' conductor 80 which are 180` degrees out of phase with the voltages in circuit 19.

The reference character S4 indicates a switch yof any suitable type, comprising wipers or brushes 8| 82 and 83, each of which has a bank of eleven contacts associated therewith. The wipers are mounted on a common shaft 'and may be simultaneously advanced from contact to contact by means of a suitable knob or dial.

'I'he multivibrator MV comprises a double tri,- ode 81, suitable grid and 'plate resistors, and plate-to-grid' coupling capacities which are variable by means of the wipers r82 and 83 of switch S4. As can be clearly seen from the drawing, the wipers complete the plate-to-gridkcoupling circuits through a different pair of condensers in each position. The control circuit 19 includes the resistor 84. The control circuit 86 includes the resistor 85 and is adapted to be connected either to conductor 80 or to control circuit 19 by means of wiper 8i of switch S4.

The following table shows the condenser capacity used at each switch position, the division factor at each switch position, and the frequency range used at each position. The values given are not critical, in View of the wide limits of the multivibrator.

Switch Capacity Division Range In order to explain how the equipment is used, it will .be assumed that an output frequency of 75 cyclesper second is desired. Inspection of the table or chart shows that this frequency lies within the range '7l-91 cycles, corresponding to the ninth switch position. The switch S4 is accordingly set at position #9. The operator now notes from the table that in position #9 the multivibrator divides by 4 and adjusts the lbeat frequency of the beat frequency oscillator until it equals the product of the desired frequency and the division factor, which is 300 cycles per second. This secures the desired output frequency, for if the applied control voltages have a frequency of 300 cycles per second, and if the multivibrator divides by 4, the output frequency will be cycles per second. Asvanother example, an output frequency of 44.75 cycles per second may be obtained by setting the switch to position #3 and by then adjusting the beat frequency to 313.25 cycles per second.

The equipment is exceptionally reliable due in part to the small frequency ranges that are actually used as compared to the ranges shown by test to be available at the various switch positions. For instance, in position #1, where the multivibrator has the smallesty limits, the range shown by test to be available isy about 1 1 cycles,

although a range of only 2 cycles is used. It will be understood, Qf, course,th at they range selected for usent "eachvswitcnaosinon should l centerl'at about 'the lsarne'biilt as the"v available range what is lconsiuerednew4 alfa {aes'ie to have protected by Letters Patent is' pointed out inthe "appended claims.l

liWhatisclairrxediszf 4 1. In a'frequencydivider,` a plurality of'multi- `vibrators arranged'in tandem, each said multivibrator 'comprising 'two space' discharge devices andv each dischargedevice comprising cathode -`means', a grid, an'dA a platefresist'ances in the cathode plate circuits of saidfdevicesall having approximately the same value, resistances in` the cathode gridcircuits of said. devices all having approximately the same value, condensers in eachY multivibrator by means of which the'plate `of each device is coupled to the grid of the other device, and means whereby each multivibrator except the last controls the next adjacent multivibrator at a sub-harmonic frequency, said last means comprising circuit connections conductive to direct current extending between adjacent multivibrators, the condensers in each multivibrator having a capacity value which bears a predetermined relation to its operating frequency when under control as set forth.

2. A frequency divider comprising a plurality of multivibrators arranged in tandem, each multivibrator including two space discharge devices each having cathode, grid, and plate elements and a condenser for coupling its plate to the grid of the other device, circuit means for transmitting control voltages of substantiallyyunaltered wave form from each preceding mult/ivibratorto each succeeding multivibrator to operate said succeeding multivibrators at progressively lower sub-harmonic frequencies, tuning elements in each succeeding multivibrator including said condensers whereby each multivibrator is tuned to a natural frequency enough lower than its operating frequency so that the natural frequency can never equal the operating frequency notwithstanding normal variations in the values of said elements, and means in each discharge device of each succeeding multivibrator for maintaining potential conditions on its grid such that the discharge device can amplify control voltages to initiate transients in the associated discharge device, the amplification factor being high enough so that transients are initiated at said sub-harmonic frequencies notwithstanding kthe fact that the multivibrators are tuned to lower frequencies. l

3. In a frequency divider, a multivibrator comprising two space discharge devices each having cathode, grid, and plate elements and a condenser for couplingv its plate to the grid of the other device, a plate resistor in the cathode plate circuit of each device, a grid resistor in the cath'- ode grid circuit of each device, means for generating control voltages at a given frequency, control circuits over which said voltages are transmitted to said multivibrator to control the same at a sub-harmonic frequency, each said coupling condenser having a value which is approximately in the center of the range over which the value can be varied while the multivibrator operates at said sub-harmonic frequency and while the values of the plate and grid resistors and the control voltages remain unchanged, each plate resistor and each grid resistor likewise having a value which is approximately at the center 4. In combination, a controlled multivibrator comprising two triodes and having two incoming control circuits terminating in said triodes, respectively, each triode having elements so proportioned that it is adapted to amplify impulses received over its associated control circuit at times when the other triode is approaching a transient, means comprising a second multivibrator for generating and transmitting positive and negative impulses alternately over each control circuit, said control circuits having characteristics such that said impulses reach the controlled multivibrator substantially unchanged in wave form, means including coupling condensers whereby each triode transmits amplified positive impulses derived from negative impulses received over its associated control circuit to the other triode to initiate transients therein, the amplified positive impulses derived from negative impulses received over one control circuit being opposed or supplemented by impulses received over the other control circuit depending on whether .the impulses in the two control circuits are in phase or not, 'and the amplification factor of said triodes being high enough so that at times when the amplified positive impulses are opposed by other impulses they still have sufficient amplitude to initiate transients as set forth.

5. In combination, a multivibrator comprising two triodes, each triode havingrmeans for amplifying control impulses at times when the grid in the adjacent triode vis rising in potential, coupling lcircuits over which the amplified impulses are transmitted from each plate to the grid of the opposite triode, whereby regenerative impulses are produced if an amplified impulse raises the potential of the grid to which it is transmitted high enough to start current flow, said regenerative impulses being out of phase with the kcontrol impulses, and means for transmitting to said multivibrator control impulses of sufficiently long duration so that the regenerative impulses produce a cumulative effect thereon notwithstanding the difference in phase.

6. In a frequency divider, a high frequency multivibrator, a low frequency multivibrator, each multivibrator comprising two vspace discharge devices each having its plate coupled to the grid of the other device by means of a condenser, two control circuits extending between said multivibrators by means of which the low frequency multivibrator is caused to operate at a sub-harmonic frequency, said control circuits including means for transmitting control voltages without substantially decreasing the duration thereof, means for changing the phase relation between the voltages carried by said control circuits -to change the operating frequency of the lowfrequency multivibrator from an even subharmonic to an odd sub-harmonic, or vice versa, and tuning elements in said low frequency multivibrator having such values that its natural frequency is lower than the lowest of said subharmonic frequencies.

7. In a frequency divider. a multivibrator comprising a pair of space discharge devices having their plate and grid elements cross coupled by condensers, two control circuits extending to said multivibrator, a second multivibrator for transmitting control voltages`, means including said k control circuits for transmitting said control voltages to said rst multivibrator in substantially unchanged wave form, resistors in said control circuits for limiting said voltages while maintaining their wave form.'and circuit elements in said nrst multivibrator having constantssuch` that each discharge device is adapted to convert negative half waves of control voltage into amplified positive impulses for the `initiation' of transients in the other discharge device, said impulses having sufficient amplitude to initiate the transients far enough in advance of the time they would occur if the multivibrator were uncontrolled to cause the multivibrator to operate at a frequency which is at least twenty-five percentH capacity means in the plurality ofmultivibrators being proportioned so that the multivibrators may produce waves of harmonically related fre-.

quencies, and coupling means between the plates of one multivibrator and the plates of another vibrator for passing to said other multivibrator a wave having substantially the same shape as a the wave produced by said one multivibrator.

9. In a frequency divider, a multivibrator comprising two space discharge devices each having cathode, grid, and plate elements anda con# denser for coupling its plate to the grid ofthe other device, two control circuits incoming to said discharge devices, respectively, tuning elements in said multivibrator having values such that the natural frequency of said multivibrator is substantially lower than the desired operating frequency, each said discharge device constituting means for amplifying control'impulses received over its associated control circuit and for trans-- mitting such amplified impulses to the other discharge device through its coupling condenser, whereby regenerative impulses are produced in ease an amplified impulse causes the device to which it -is transmitted to become conductive, said regenerative impulses beingv out of phase with the initial control impulse to which they are responsive, and means for transmitting over said control circuits control impulses having a frequency which is a multiple of said desired operating frequency and which are of suillciently long duration so that the regenerative impulses resulting from any control impulse produce a cumulative effect thereon notwithstanding the difference in phase.

THOMAS B. GIBBS. MORRIS'E. BROWN. PARKER B. WICKHAM. 

